Lost wax casting has been used in the past to produce propellers for marine engines. In the typical lost wax process, a wax propeller pattern is die cast and has a configuration identical to that of the metal propeller to be produced. The wax pattern is immersed in a ceramic bath and then dipped in sand to provide a stucco-like coating on the pattern. Six to eight coats of the sand/ceramic coating are applied to the pattern to provide a ceramic shell. The composite structure is then heated to a temperature sufficient to melt the wax, and after draining of the wax, the ceramic shell is fired at an elevated temperature generally in the range of about 1700.degree. F. After removal of the ceramic shell from the furnace, the molten metal is immediately poured into the shell to provide the cast metal - propeller.
Due to the configuration of the propeller and the pitch of the blades, one manner of producing the wax propeller pattern, as practiced in the past, was to employ a die that was split circumferentially of the hub and the die sections were pulled axially after casting of the wax pattern. With this die construction, the outwardly flared end on the hub could not be cast integrally with the remainder of the hub, so that the flared end was separately cast and then bonded to the remainder of the hub. To join the flared end to the hub, the abutting edges were provided with interlocking recesses and wax was then used to join the members. The resulting joint was then manually blended or feathered to provide a smooth contour between the hub and the flared end.
It is also desirable to be able to cast a logo into the hub of the propeller. However, when using axially drawn die sections, a logo could not be cast in the hub. Furthermore, the use of axially drawn die sections rquired that the hub have a slight draft as opposed to a pure cylindrical contour.
In order to overcome these problems, a modified die assembly was utilized for die casting wax propeller patterns. In the modified construction, three separate inserts were assembled from the top into the outer die sections, with each insert defining a third of the hub without blades. The blades in this die assembly were defined by the upper and lower dies. After casting, the upper and lower dies were moved axially and the inserts were then manually removed. This modified method enabled a flared end to be formed integrally with the hub, but resulted in the deposit of wax at the joint between the blades and the hub, which again had to be manually blended or feathered. In this process, the manually positioning of the inserts was a difficult and time consuming task, resulting in a substantial set-up time. In addition, the use of the separate inserts substantially increased the overall cost of the die assembly, because of the added number of movable die elements.
Another method used in the past to produce wax propeller patterns incorporated three separate die inserts, each mounted for sliding movement along horizontal guide tracks from an outer open position to an inner closed position where the inserts defined a die cavity in combination with a central core. In this construction, the die inserts were held in the closed position by angular wedge members that were carried by the press and engaged the outer surfaces of the inserts to wedge the inserts to the closed position. While this prior unit had the capability of casting the outwardly flared end of the hub integrally with the remainder of the hub, the die inserts had to be manually moved between the open and closed positions, and the core had to be manually stripped from the pattern with the result that the die casting operation had a relatively long cycle time.
In practice, a marine engine manufacturer may require a number of different families of propeller designs, with each family differing from other families by size, and each propeller within a given family differing by blade pitch. This can result in a total of perhaps 80 to 100 different propeller designs.
In the past, a separate and complete die assembly was required for each propeller design and thus the die cost for producing the wax propeller patterns was very substantial. Moreover, each complete die assembly could not be transported or handled by a single workman, with the result that it was necessary to use mechanical equipment, such as a fork lift truck or a hoist, to move the die assemblies into and out of the press.
Because of these problems there has been a distinct need for a die assembly for producing wax propeller patterns which is less costly, has a reduced set-up time and can be installed without the need of mechanical equipment.